### Abstract

The molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C_{2v}, D _{3h}, and D_{2h} symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of C_{s}, C_{3v}, and C_{2v} symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 Å per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C≡C bond of the ethynyl group is unaffected by the pattern of substitution, the C_{ipso}-C_{ethynyl} bonds in p-diethynylbenzene are 0.001-0.002 Å shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the C_{ipso}-C _{ethynyl} and C_{ipso}-C_{ortho} bonds in the three molecules, 0.023-0.027 Å, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level.

Original language | English |
---|---|

Pages (from-to) | 2045-2052 |

Number of pages | 8 |

Journal | Journal of Physical Chemistry A |

Volume | 110 |

Issue number | 5 |

DOIs | |

Publication status | Published - Feb 9 2006 |

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### ASJC Scopus subject areas

- Physical and Theoretical Chemistry

### Cite this

*Journal of Physical Chemistry A*,

*110*(5), 2045-2052. https://doi.org/10.1021/jp058174w

**Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chemical calculations.** / Campanelli, Anna Rita; Arcadi, Antonio; Domenicano, Aldo; Ramondo, Fabio; Hargittai, I.

Research output: Contribution to journal › Article

*Journal of Physical Chemistry A*, vol. 110, no. 5, pp. 2045-2052. https://doi.org/10.1021/jp058174w

}

TY - JOUR

T1 - Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chemical calculations

AU - Campanelli, Anna Rita

AU - Arcadi, Antonio

AU - Domenicano, Aldo

AU - Ramondo, Fabio

AU - Hargittai, I.

PY - 2006/2/9

Y1 - 2006/2/9

N2 - The molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C2v, D 3h, and D2h symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of Cs, C3v, and C2v symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 Å per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C≡C bond of the ethynyl group is unaffected by the pattern of substitution, the Cipso-Cethynyl bonds in p-diethynylbenzene are 0.001-0.002 Å shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the Cipso-C ethynyl and Cipso-Cortho bonds in the three molecules, 0.023-0.027 Å, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level.

AB - The molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C2v, D 3h, and D2h symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of Cs, C3v, and C2v symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 Å per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C≡C bond of the ethynyl group is unaffected by the pattern of substitution, the Cipso-Cethynyl bonds in p-diethynylbenzene are 0.001-0.002 Å shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the Cipso-C ethynyl and Cipso-Cortho bonds in the three molecules, 0.023-0.027 Å, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level.

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U2 - 10.1021/jp058174w

DO - 10.1021/jp058174w

M3 - Article

VL - 110

SP - 2045

EP - 2052

JO - Journal of Physical Chemistry A

JF - Journal of Physical Chemistry A

SN - 1089-5639

IS - 5

ER -